This project has progressed very little this year. A simple package of software developed to simulate physiologically based models is being tested on elementary models. An apparatus for measuring partition coefficients has been built and is being tested. Partition coefficients for arsine will be measured so that a simulation can be used in conjunction with the arsine studies which are in progress. A collaboration with the proposed skill center on physiologically based modeling is planned in conjunction with a two-year methylene chloride study. A closed exposure system will be used to measure global pharmacokinetic parameters at various time points during the study. The effects of changes in these parameters will then be evaluated using existing models.